This invention relates to a process for the beneficiation of coal and solid carbonaceous fuel materials, and more particularly to an improved process for the beneficiation and enhanced recovery of coal.
Known resources of coal and other solid carbonaceous fuel materials in the world are far greater than the known resources of petroleum and natural gas combined. Despite this enormous abundance of coal and related solid carbonaceous materials, reliance on these resources, particularly coal, as primary sources of energy, has been for the most part discouraged. The availability of cheaper, cleaner burning, more easily retrievable and transportable fuels, such as petroleum and natural gas, has in the past, cast coal to a largely supporting role in the energy field.
Current world events, however, have forced a new awareness of global energy requirements and of the availability of those resources which will adequately meet these needs. The realization that reserves of petroleum and natural gas are being rapidly depleted in conjunction with skyrocketing petroleum and natural gas prices and the unrest in the regions of the world which contain the largest quantities of these resources, has sparked a new interest in the utilization of solid carbonaceous materials, particularly coal, as primary energy sources.
As a result, enormous efforts are being extended to make coal and related solid carbonaceous materials equivalent or better sources of energy, than petroleum or natural gas. In the case of coal, for example, much of this effort is directed to overcome the environmental problems associated with its production, transportation and combustion. For example, health and safety hazards associated with coal mining have been significantly reduced with the onset of new legislation governing coal mining. Furthermore, numerous techniques have been explored and developed to make coal cleaner burning, more suitable for burning and more readily transportable.
Gasification and liquefaction of coal are two such known techniques. Detailed descriptions of various coal gasification and liquefaction processes may be found, for example, in the Encyclopedia of Chemical Technology, Kirk-Othmer, Third Edition (1980) Volume 11, pages 410-422 and 449-473. Typically, these techniques, however, require high energy input, as well as the utilization of high temperature and high pressure equipment, thereby reducing their widespread feasibility and value.
Processes to make coal more readily liquefiable have also been developed. One such process is disclosed in U.S. Pat. No. 4,033,852 (Horowitz, et al) incorporated by reference herein. This process involves chemically modifying the molecular structure of the coal, the effect of which alters its morphology and thus renders a portion of the coal more readily liquefiable than the natural forms of coal.
In addition to gasification and liquefaction, other methods for converting coal to more convenient forms for burning and transporting are also known. For example, the preparation of coal-oil and coal-aqueous mixtures are described in the literature. Such liquid coal mixtures offer considerable advantages. In addition to being more readily transportable than dry solid coal, they are more easily storable, and less subject to the risks of explosion by spontaneous ignition. Moreover, providing coal in a fluid form makes it feasible for burning in conventional apparatus used for burning fuel oil. Such a capability can greatly facilitate the transition from fuel oil to coal as a primary energy source. Typical coal-oil and coal-aqueous mixtures and their preparation are disclosed in U.S. Pat. Nos. 3,762,887, 3,617,095 and 4,217,109 and British Pat. No. 1,523,193.
Regardless, however, of the form in which the coal is ultimately employed, the coal must be cleaned because it contains substantial amounts of sulfur, nitrogen compounds and mineral matter, including significant quantities of toxic metal impurities. During combustion these materials enter the environment as sulfur dioxides, nitrogen oxides and compounds of toxic metals. If coal is to be accepted as a primary energy source, it must be cleaned to prevent pollution of the environment.
Accordingly, chemical as well as physical coal cleaning (beneficiation) processes have been extensively explored. In general, physical coal cleaning processes involve pulverizing the coal to release the impurities, wherein the fineness of the coal generally governs the degree to which the impurities are released. However, because the costs of preparing the coal rise exponentially with the amount of fines produced, there is an economic optimum in size reduction. Based on the physical properties that effect the separation of the coal from the impurities, physical coal cleaning methods are generally divided into four categories: gravity, flotation, magnetic and electrical methods.
In contrast to physical coal cleaning, chemical coal cleaning techniques are in a very early stage of development. Known chemical coal cleaning techniques include, for example, oxidative desulfurization of coal (sulfur is converted to a water-soluble form by air oxidation), ferric salt leaching (oxidation of pyritic sulfur with ferric sulfate), and hydrogen peroxide-sulfuric acid leaching. Other methods are also disclosed in the above-noted reference to the Encyclopedia of Chemical Technology, Vol. 6, pages 314-322.
A recent promising development in the art of chemical coal beneficiation is disclosed in copending U.S. patent applications Ser. Nos. 114,357 and 114,414, each filed Jan. 22, 1980 and in U.S. Government Report No. 2694 entitled "Fuel Extension by Dispersion of Clean Coal in Fuel Oil" all incorporated herein by reference. In summary, according to this coal beneficiation process, coal is first cleaned of rock and the like and pulverized to a fine size. The pulverized coal, now in the form of a water slurry, is then contacted with a mixture comprising a polymerizable monomer, and a polymerization catalyst. The resultant surface treated coal is highly hydrophobic and the oleophilic and is thus readily separated from unwanted ash and sulfur using oil and water separation techniques. Moreover, the hydrophobic coal can be readily further dehydrated to very low water levels without employing costly thermal energy. The clean, very low moisture content coal resulting from this process can then be employed as is, i.e., as a dry solid product, or further processed to advantageous coal-oil or coal-aqueous mixtures.
In any coal cleaning process, whether it be physical or chemical, the recovery of maximum yields of useable product is always an important economic concern. Much effort has also been devoted to the recovery of the huge amounts of residual coal fines which are generated during all coal beneficiation processes.
One recovery method, for example, which has been developed is the froth flotation process which consists of agitating the finely divided coal and mineral suspension with small amounts of frothing reagents in the presence of water and air. The frothihg reagents assist the formation of small air bubbles that collect the hydrophobic coal particles and carry them to the surface. The hydrophilic mineral matter is wetted by water and drawn off as tailings. However, the recovery of fine coal residuals in coal cleaning refuse water using conventional froth flotation is so poor that the procedure is seldom practiced commercially.
On the other hand, U.S. patent application, Ser. No. 06/230,056, filed concurrently herewith discloses a desirable integrated process for the beneficiation and enhanced recovery of product including the recovery of residual coal particles lost in prior art procedures.
A further problem associated with the beneficiation and recovery of coal is the variety of kinds of coal that are naturally found. That is, for example, it is well known that the so-called low rank coals do not respond well to beneficiation using conventional flotation processes, particularly because the beneficiated products do not float well. However, a desirable and improved process for beneficiating and recovering low rank coal is disclosed and claimed in U.S. patent application, Ser. No. 06/230,060 filed Jan. 29, 1981.
Accordingly, other methods for the improved beneficiation and enhanced recovery of coal, including residual coal fines, would obviously be very desirable. Of particular importance is a method which not only provides for an improved beneficiated product in high yield but also which is suitable for the beneficiation and good recovery of low rank coals not normally suitable for beneficiation. In addition, other processes for the liquefaction of coal would also be welcome, not to mention the desirability for a process which has the capability of achieving all the aforementioned goals but also provides for the liquefaction of coal and moreover the recovery of clean liquefied coal product.